1. Field of the Invention
Aspects of the present invention relate to a paper supply device to supply printable medium to a main body of an image forming apparatus, and an image forming apparatus having the same.
2. Related Art
An image forming apparatus, such as a copying machine, a fax machine or a printer, includes a paper supply device to supply paper into a main body. Typically, the paper supply device picks up the loaded paper sheet by sheet, and supplies the paper into a main body of the image forming apparatus. As used herein, “paper” refers to any medium onto which an image may be printed, including paper, thermal paper, and transparent film.
Japanese Laid-Open Patent Publication No. 2006-8343 (published on Jan. 12, 2006) discloses an image forming apparatus with a paper supply device. The paper supply device of the disclosed image forming apparatus includes a pickup roller to pick up the paper loaded in a paper cassette, a paper supply roller, and a spring to push up a paper supply plate loaded with the paper toward the pickup roller. The paper supply device further includes a lifting device, which, if the amount of loaded paper decreases as the paper is supplied to the main body, moves the paper supply plate toward the pickup roller so that the uppermost paper contacts the pickup roller.
The lifting device includes a planetary gear coupled to the paper supply plate, an internal gear engaged with the planetary gear and coupled to a frame of the paper supply device, a sun gear engaged with the planetary gear, a ratchet rotating coaxially with the sun gear, and a restraining member restraining or releasing the ratchet. The restraining member includes a locking lever that is latched by the ratchet, a spring biasing the locking lever, and a solenoid device that releases the locking lever from the ratchet.
The pickup roller and the paper supply roller are rotatably supported by a pickup frame in the main body. The pickup frame moves up and down within a predetermined region so that a position of the pickup roller changes according to a height of the loaded sheets of paper. A height sensor is mounted near the pickup frame to sense a height of the pickup frame. The solenoid device of the restraining member is controlled based on a sensing signal from the height sensor for the position of the pickup frame. If the pickup roller moves down, the solenoid device operates the locking lever to be released from the ratchet, and the paper supply plate is biased upward by an elastic force of the spring mounted below. The planetary gear moves up by revolving around the sun gear by the rotation of the sun gear, thereby moving up the paper supply plate.
The loaded sheets of paper move up by the lifting force of the paper supply plate. If the uppermost paper pushes up the pickup roller, the pickup frame also moves up. If the pickup frame moves up, the height sensor senses the movement of the pickup frame, and the operation of the solenoid device is stopped, so that the locking lever restrains the ratchet. If the ratchet is restrained, the rising of the paper supply plate is stopped.
The above conventional paper supply device is configured such that when the paper is picked up, the rising force created by the spring pushing up the paper supply plate is applied to the pickup roller through the loaded sheets of paper. However, because the rising force changes according to the amount of loaded sheets of paper on the paper supply plate, a pickup force is not constant. Because the rising force of the spring changes according to the amount of loaded sheets of paper and the weight of the loaded sheets of paper, the pickup force is not constant.
Further, because the lifting device to push up the paper supply plate includes the internal gear, the planetary gear, the sun gear, the ratchet, the locking lever, the solenoid device, the height sensor, etc., the above conventional paper supply device has a great number of components and complicated constitution of the components.